In pelvic treatments, in particular hip replacement operations, patients typically are operated on while lying on their side, and this requires the patient to be fixed such that their pelvis is positioned at rest. This is conventionally achieved using patient positioners, which comprise supporting means for the lower region of the patient's body. In this position, however, it is often difficult to ascertain navigation information by tapping body landmarks (ASIS and pubic points) within the framework of medical navigation, because these characteristic pelvic landmarks are in most cases difficult to access, at least in the lower region. X-ray or fluoroscopic registration is therefore often employed.
Many conventional patient positioners use pads or poles that are fixed to an arm system of the operating table or to similar fixed means. Such a mechanical positioner is known, for example, from U.S. Pat. No. 6,311,349.
In addition to mechanical positioners, there are also positioners that comprise navigation reference means for medical navigation; such a positioner is known, for example, from WO 2004/089192 A2. The reference means are used to indirectly localize the ASIS and pubic points, which can be used to register the front pelvic plane.
When registering the ASIS points and pubic points by manually tapping the points and access to these points is impeded by the presence of a positioning means, it is necessary to fall back on fluoroscopic registration. There are positioning means for this purpose that are made of a material that is as radiolucent as possible and comprise the supporting devices already mentioned above, e.g., poles and pads, which are fixed to a carrier system (on a guiding rail) of the operating table. A problem with these existing positioning means for navigation software arises when both fluoroscopic images are to be recorded for registration and image-free registration is to be performed (e.g., by moving a navigation pointer to said points). Despite the at least largely radiolucent material used in the positioning means, it still can cause shadows and edges on the images. On the x-ray recording 60 in FIG. 7, for example, a region 61 is marked that shows the shadow of a patient positioner, which is hiding the left-hand os pubis structure. FIG. 8 shows a fluoroscopic recording 62 obtained using a “radiolucent” positioner, which also shows how the shadow 63 of a part of the positioner hides the content of the image and weakens the contrast.
Another problem with the known positioning devices for lateral positioning arises when adipose patient tissue overlaps the pelvic rim marks. This makes it difficult or impossible to tap these points with a navigation pointer. Moving a pointer to the pelvic landmarks in this way also is often obstructed by the poles and pads. For this reason, the patient is initially positioned supine, registration is performed and the patient is then repositioned laterally. This is not only time-consuming but also critical with regard to sterility, which may be lost as result of repositioning.
All the aforementioned disruptions to the registration procedure, e.g., a lack of or incorrect image-free tapping with a pointer or a misinterpretation of fluoroscopic images due to shadows or a lack of contrast, can lead to incorrect registration and therefore errors in navigation, which can in turn have a negative effect on the treatment result.